1. Field of the Invention
The present invention relates to a pouch type secondary cell with high water-resistance.
2. Discussion of the Related Art
A secondary cell has a structure in which the electrode assembly of a positive electrode, a separation layer, and a negative electrode which can be charged or discharged is embedded in the casing of a laminate sheet, including a metal can or a resin layer and a metal layer, such as a cylinder type or an angular type, with an electrolyte impregnated therein. The electrode assembly can be classified depending on its structure. The electrode assembly can include, for example, a cylinder type (i.e., a take-up type) to Jelly-roll type electrode assembly, a stack type electrode assembly in which a number of positive electrodes and negative electrodes cut in a specific size unit are sequentially stacked with a separation layer interposed therebetween, a stack/folding type electrode assembly in which bi-cells or full cells, each having positive electrodes and negative electrodes of a specific unit stacked therein with a separation layer interposed therebetween, are taken up, etc.
From among the cell structures, the cylinder type cell structure is advantageous in that it is excellent in the structural stability, and the cell structure using the laminate sheet casing are advantageous in that it is light in weight and can be easily manufactured. Recently, the use of the cell using the laminate sheet is increased suddenly in accordance with a tendency toward the downsizing, light weight, and thinness of electrons devices and a need for a reduction in the weight of medium and large-sized cell packs. The cell using the laminate sheet is frequently called a pouch type secondary cell because of the shape of a casing.
FIG. 1 is a dismantled perspective view which is pertinent to the manufacture of one secondary cell using a laminate sheet as a cell casing (hereinafter referred to as a ‘pouch type secondary cell’).
Referring to FIG. 1, the pouch type secondary cell 100 is manufactured by mounting an electrode assembly 300, formed of a positive electrode, a separation layer, and a negative electrode, on a pouch type cell casing 200 formed of a laminate sheet made of polymer resin and aluminum (Al) and then coupling electrode leads 410 and 420 to the cell casing 200 with them exposed at the top of the cell casing 200.
The cell casing 200 includes an upper cover 230 and a lower casing 220 having a reception unit 210 formed therein. The cell casing 200 has a folder type structure having a bottom integrated.
In the state in which the electrode assembly 300 is seated in the reception unit 210, the top surface 240 of the lower casing 220 and the outer circumferential surfaces 250 on both sides of the lower casing 220 are adhered to the contact surface of the upper cover 230 and sealed together. Accordingly, after the cell is assembled, the top surface 240 of the lower casing 220 and the outer circumferential surfaces 250 on both sides of the lower casing 220 form a sealing unit.
Electrode tabs 310 and 320 protruded from the electrode assembly 300 are connected to the respective electrode leads 410 and 420. Sealing films 500 are connected to respective portions where the cell casing 200 and the electrode leads 410 and 420 are connected to each other. The sealing films 500 function to prevent the leakage of an electrolyte, prevent moisture in air from infiltrating the cell, and guarantee the electrical insulating property of the electrode leads 410 and 420.
FIG. 2 shows an example of another pouch type secondary cell and it is a perspective view showing the pouch type secondary cell in which electrode leads are respectively protruded from the top and bottom of a cell casing.
The pouch type secondary cell 101 of FIG. 2 differs from the pouch type secondary cell 100 of FIG. 1 in that the electrode leads 411 and 421 are respectively disposed at the top and bottom of the cell casing and the cell casing is separated into a lower casing 221 and an upper casing 231. Accordingly, the cell casing consists of the upper sealing unit 241, the lower sealing unit 261, and the sealing units 251 and 271 on both sides which are formed by thermally compressing the lower casing 221 and the upper casing 231. A reception unit 211 can be formed only in the upper casing 231 or the lower casing 221 or can be formed in both the lower and upper casings 221 and 231.
FIG. 3 shows a process of forming the sealing unit of a laminate sheet which is commonly used as a cell casing in a pouch type secondary cell and shows a cross section of the coupled laminate sheet.
Referring to FIG. 3, the laminate sheet 10 includes an external resin layer 11 forming the outermost part, a metal layer 12 preventing the penetration of materials, and an internal resin layer 13 performing a sealing function.
The external resin layer 11 functions to protect the cell from the outside and so requires the thickness versus an excellent tension strength, atmosphere corrosion resistance, etc. The external resin layer 11 is commonly made of flexible nylon. The metal layer 12 functions to prevent air, moisture, etc. from being introduced into the cell and it is commonly made of aluminum (Al). The internal resin layer 13 is thermally compressed by heat and pressure applied in the state in which an electrode assembly is built in the cell casing, thus providing a sealing property. The internal resin layer 13 is commonly made of cast polypropylene (CPP).
The cell casing sheet 10 of the multi-layer laminate structure is configured to have the internal resin layers 13 facing each other in the sealing unit. The internal resin layers 13 are coupled together by thermal compression. In this case, the internal resin layer 13 is exposed externally at a portion where the laminate sheets are coupled together. Moisture can easily penetrate into the exposed internal resin layer 13 because it is commonly made of polymer resin. The penetrated moisture has bad problems in terms of cell safety, such as that generates a side reaction within the cell, reduces the life span of the cell, oxidizes the metal layer 12 of the cell casing, weakens the adhesion strength of the sealing unit, and possibly leaks the electrolyte. Further, if the cell is used for a long time, the penetrated moisture reduces the life span and safety of the cell. Accordingly, various attempts to prevent the penetration of moisture and the leakage of the electrolyte had been made.
For example, Japanese Unexamined Patent Application Publication No. 2004-087239 discloses a laminate sheet in which an internal resin layer is coated with metal layers by shaping lateral portions where laminate films are adhered together through thermal pressurization and compression so that the metal layers (i.e., shut-off metal layers) are brought into contact with each other.
In the above technique, however, the metal layers are simply brought into contact with each other through thermal pressurization and compression. Accordingly, the above technique is problematic in that sufficient water-resistance cannot be obtained because the coupling of the metal layers is not robust and the metal layers are separated from each other because the coupling of the metal layers is weakened because of a long-term use.
Japanese Unexamined Patent Application Publication No. 2004-055154 discloses a technique for preventing the leakage of an electrolyte and the penetration of moisture resulting from the exposure of an inner resin layer by extending the peripheral portion of one of a pair of laminate films more outward than the peripheral portion of the other of the pair of laminate films and performing laser welding on a portion with which the front end of the extended portion, bent toward the peripheral portion of one of the pair of laminate films, is brought into contact so that the outermost layer is volatilized by the heat of the laser light and the ends of both metal layers are melted and combined together.
However, the above method is problematic in that it requires a high degree of accuracy in terms of the process because one of the pair of laminate films must be accurately bent enough to seal the other of the pair of laminate films and the cell manufacture costs are increased because the process is added.
Further, Japanese Unexamined Patent Application Publication No. 2000-223090 discloses a technique in which a metal layer and a thermal compression layer are stacked, part of a thermally compressed portion toward the inside of a cell casing is removed to expose the metal layer, and dual sealing processing, including the thermal compression of the thermal compression layer and the welding of the metal layer, is performed when a cell is sealed.
However, the above method is problematic in that the process is complicated and the costs are increased because the dual sealing processing is performed.